The present invention relates to an inductive plasma mass spectrometer which ionizes a sample with inductive plasma and detects the ions thus generated by mass-separating them using a mass filter to identify and quantify minor impurities in the sample.
Conventional inductive plasma mass spectrometers have a general configuration as shown in FIG. 2.
In FIG. 2, 1 designates a plasma torch; 2 designates an inductive coil; 3 designates inductive plasma. A plasma generating portion is constituted by the plasma torch 1, the inductive coil 2 and gas (e.g., Ar) flowing into the plasma torch 1. A liquid sample is normally nebulized by a nebulizer (not shown) and is supplied to the plasma torch 1 along with carrier gas. The inductive coil 2 is wound around the forward end of the plasma torch 1 and high frequency power of 27.12 MHz or 40 MHz is applied thereto. The inductive plasma 3 is maintained by being inductively coupled with this high frequency power. Such a plasma generating portion is referred to as inductively coupled plasma (ICP) while there are instances wherein a microwave inductive plasma (MIP) such as that disclosed in Japanese patent publication No. H03-95899 is used as a plasma generating portion.
4 designates a sampling cone. 5 designates a skimmer cone. 6 designates a vacuum chamber. 7, 8, and 9 designate pumps a, b, and c, respectively. The sampling cone 4 and the skimmer cone 5 are in the form of a cone and have round holes of diameters on the order of 0.9 to 1.2 mm and 0.3 to 0.6 mm, respectively. The sample which has been ionized in the inductive plasma 3 are introduced into the vacuum chamber 6 through the holes of the sampling cone 4 and the skimmer cone 5.
The vacuum chamber 6 is differentially evacuated by a pump 7, a pump 8, and a pump 9 to maintain vacuum therein. As to the types of the pumps, normally, a rotary pump is used as the pump and turbo-molecular pumps or oil diffusion pumps are used as the pumps b and c of 8 and 9, respectively.
Although not shown in the figure, auxiliary pumps are normally used for the pumps 8 and 9. The vacuum evacuator is constituted by the pumps 8 and 9 and the auxiliary pumps.
10 designates a mass filter. 11 designates an ion lens. 12 designates a repeller electrode. 13 designates a detector. The mass filter 10 allows ions having a mass corresponding to preset voltage to pass therethrough from among the ions incident thereon and a quadrupole mass spectrometer is normally used. The ion lens 11 efficiently causes the ions which have passed through the skimmer cone 5 to enter the mass filter 10 and to block the light from the plasma 3.
The ions which have passed through the mass filter 10 are detected by the detector 13. The CHANNEL TRON from GALILEO CORPORATION is normally used as the detector 13. The repeller electrode 12 leads the ions which have passed through the mass filter 10 to the ion detector 13. Positive voltage is applied when the ions are positive ions and negative voltage is applied when the ions are negative ions.
The mass number of impurities in the sample is obtained from the voltage applied to the mass filter 10 when they are detected by the detector 13. In other words, the impurities are identified. Further, the concentration of impurities can be obtained from the quantity of ions detected by the detector 13.
Next, the configuration from the exit of the mass filter 11 up to the detector 13 will now be described with reference to FIG. 3. In FIG. 3, 10 designates a mass filter; 12 designates a repeller electrode; 13 designates a detector; 14 designates an extraction electrode; 15 designates light; and 16 designates ions.
Ions which have reached the exit of the mass filter 10 are extracted from the mass filter 10 by the extraction electrode 14 and are accelerated. The repeller electrode 12 and the detector 13 are disposed opposite to each other about the central axis of the mass filter 10. This is to allow only the ions which have passed through the mass filter 10 to reach the detector 13.
Specifically, the ions 16 are led to the detector 13 by an electric field which is produced by electric potential applied to the repeller electrode 12 and electric potential applied to be detector 13 while the light 15 (which has not been blocked by the ion lens shown in FIG. 2) acting as a background noise is left traveling straight so that it will not reach the detector 13.
17 designates a negative high voltage power source. 18 designates a pulse pre-amplifier. 19 designates a discriminator.
20 designates a data processor. 21 designates an output terminal. When the ions detected are positive ions, a high voltage of about -2 to -2.5 KV is applied from the high voltage power source 17 to the forward end of the detector 13, the rear end thereof being grounded. The ions 16 incident on the detector 13 are charge-amplified by a factor of about 10 to 10 in the detector 13 and reach the output terminal 21 in the form of pulses.
The pulses at the output terminal 21 are amplified by the pulse pre-amplifier by a factor of about 10 to 100 and are sent to the discriminator 19. The pulses include noises. The discriminator 19 separates a ion signal from the noises according to the pulse height and sends only the ion signal to the data processor 20. The data processor 20 counts the pulses as the ion signal.
The mass, i.e., identification of the ions is performed according to the voltage at the mass filter 10 at the time of counting and the concentration of minor impurities is obtained from the quantity counted or the counting rate. When negative ions are detected, the polarities of the voltage applied to the repeller electrode 12 and the forward end of detector 13 are reversed. Description will be omitted in this regard because it is disclosed in, for example, an article "Channel electron murutipliers" in "Reprinted from American Laboratory, March, 1979.
According to such a detection method, the range of quantifiable concentrations of minor impurities is from about 1 ppq (1/10) to about 100 ppb (1/10). The reason is that if the flux of the ions incident on the detector 13 increases, electrification occurs in the vicinity of the output terminal inside the detector 13 due to emission of secondary electrons in a large amount, which results in a change in the distribution of electric potential to reduce the strength of the electrolyte, thereby suppressing amplification of subsequent signals. The method of quantifying minor impurities in a concentration of about 100 ppb (1/10) or more is known and will now be described with reference to FIG. 4.
In FIG. 4, the mass filter 10, the extraction electrode 14, and the repeller electrode 12 are similar to those previously described. 13a designates a detector. The detector 13a has a structure wherein an analog anode 22, an isolation grid 23, and a protection grid 24 are provided in the middle of its body and an output terminal 21 is provided at the rear end thereof. When positive ions are detected, negative voltage is applied by the negative high voltage power source 17 to the forward end of the detector 13a; positive voltage is applied by a positive high voltage power source 28 to the rear portion; and 0 electric potential is applied to the protection grid 24.
In this detector 13a, the charge of incident ions is amplified to about 104 at the position of the analog anode 22 and to about 10 at the output terminal 21. 25 designates a pre-amplifier. 26 designates a VF converter. 27 designates a protection circuit. If the impurity concentration to be detected is about 100 ppb (1/10) or less (i.e., ion incidence of about 10 pulses per second or less), the charge of the ions is amplified up to the output terminal 21 and pulse-counted by the data processor 20 through the pulse pre-amplifier 18 and the discriminator 19 (Hereinafter, this is referred to as ion count method.).
When the impurity concentration to be detected is high [concentrations in the range of about 10 ppb (1/10) to 100 ppm (1/10)], the ions incident on the detector 13a are current-detected at the analog anode 22. The current flowing through the analog anode 22 is converted to voltage and amplified by the pre-amplifier 25 and is supplied to the VF converter 26. The VF converter 26 converts the voltage input by the pre-amplifier to pulses having a frequency proportionate thereto which are pulse-counted by the data processor 20a (Hereinafter, this is referred to as coulometric detection method.).
The data processor 20 determines whether to employ the ion count method or employ current detection method through management of the quantity of the input from the discriminator 19 or the VF converter 26 (counting rate). To perform the current detection, it sends a signal to the protection circuit 27 to protect the downstream of the detector 13a and applies voltage to the protection grid 24 to prevent secondary electrons from entering the downstream of the detector 13a. An inductive plasma mass spectrometer having such a configuration can measure concentrations of minor impurities in the range of about 1 ppq (1/10) to 100 ppm (1/10).
However, the prior art has the problems as follows. The first problem is the life of the detector in measuring impurities in high concentrations [concentrations on the ppm (1/10) order]. In the prior art, almost all of the ions which had passed through the mass filter 10 have always reached the detector. This is because the voltage applied to the detector 13 is 2 to 2.5 KV while the voltage applied to the repeller electrode 12 is on the order of several tens volts to 100 volts and, therefore, the electric field produced by the detector is very strong. The ions which have entered the detector stay on the inner wall of the detector, reducing the yield of the emission of secondary electrons. As a result, the life of the detector is shortened by measuring impurities in high concentrations.
The second problem originates from the current detection performed to detect impurities in high concentrations. The current detected is within a narrow range of effective figures in about four digits on the order of 1/10 to 1/10 A. For 1/10 A or more, the amplification performed by the detector is suppressed for the reason described above, and detection of very small current of 1/10 A or less suffers from reduction in accuracy (Measurement is disabled due to saturation.). Although the region of 1/10 A or less can be measured using the ion count, it is desirable as an analyzer to be capable of performing measurement at intermediate concentrations under the same conditions as for measurement at high concentrations.
The third problem is the cost. Compared to the configuration in FIG. 3, the prior art shown in FIG. 4 increases not only elements such as the pre-amplifier, VF converter, protection circuit, positive high voltage power source but also the price of the detector which is an expendable part.
The present invention has been conceived to solve the above-mentioned problems, and it is an object of the present invention to provide a detection system is an inductive plasma mass spectrometer wherein deterioration of the detector is suppressed, the range of concentrations which can be measured under the conditions for measurement at high concentrations is set to have effective figures in about six digits and which is inexpensive.
The above and other objects and novel features of the present invention will be apparent from the description in this specification and the accompanying drawings.